Several components in the automotive, electrical, agricultural, or toy industries such as alloy wheels, electronic cases, steering wheels, or compressor parts are produced in a high volume by high-pressure die casting, low-pressure casting, or gravity casting processes. In these mass production casting processes, molten metal alloys with a temperature substantially higher than the liquidus temperature are poured and cast. The operation then needs to wait for the casting to fully solidify before it can be removed from the mold or die. To speed up the solidification process, internal cooling by air or water is often applied to the die. In several cases, after the part is removed the surfaces of the die are sprayed by a cooling fluid with a mold release agent. The internal and external cooling processes of the die are used to minimize the cycle time of the process, which helps increase the productivity.
The difference between the pouring temperature and the liquidus or freezing temperature is called ‘superheat temperature’. In industrial practice, the superheat temperature is quite high, generally ranging from 80° C. to as high as 200° C. depending on the complexity, size, and section thicknesses of the casting parts. The reasons for having high superheat temperatures in the mass production casting processes are such as (1) to ensure complete filling of the die cavity, (2) to avoid metal buildup in the crucible or ladle due to non-uniform heat loss in the crucible or ladle causing die filling problem and premature solidification of some regions, which causes shrinkage porosity, (3) to allow time for complete directional solidification, which yields parts with little or no shrinkage porosity, and (4) to allow entrapped air bubbles during melt flow to escape before being trapped by solidification.
This high superheat casting processes have been well accepted and generally practiced in mass production. However, the processes lead to several cost disadvantages, which include (1) long cycle time, (2) high energy cost to melt and hold the molten metals, (3) high energy cost for the cooling water, (4) high water treatment cost from die spray, (5) high coolant and die release agent cost, and (6) high reject rates from shrinkage porosity. These disadvantages result in inefficiency of the process and increased production costs.
To solve these problems, several inventions relating to casting in semi-solid state have been proposed such as disclosed in U.S. Pat. Nos. 6,640,879, 6,645,323, 6,681,836, and EP1981668. Semi-solid metal casting involves casting of metals at a temperature lower than the liquidus or freezing temperature with some fractions of solidified solid nuclei. The pre-solidified solid nuclei help reduce turbulent flow problems and shrinkage porosity, resulting in high quality casting parts. However, due to the low casting temperature and high viscosity of the semi-solid metals, the casting processes and the die design need to be modified before the process can be applied successfully. In semi-solid metal casting, a special metal transfer unit may be needed to feed the semi-solid metals into the shot sleeve and then into the die. The die design may also need to be modified to allow complete filling of the semi-solid metals in the die cavity. Normally, thicker gates will be needed with shorter flow distances. Therefore, application of semi-solid metal in the mass production processes requires some time and investment. These semi-solid casting processes are not sufficiently cost effective so they have not been widely applied in the casting industry yet. It is, therefore, the objective of this invention to solve the disadvantages of conventional casting with high superheat temperature and semi-solid metal casting to offer cost savings in the metal casting industries with high production volume by casting molten metals at a low to zero superheat. Even though it is obvious that casting with a low to zero superheat temperature can yield several benefits, the current casting processes cannot simply apply this technique in the mass production. It is not simple to pour and cast the melt with low to zero superheat temperature without any special modifications to the casting process because it is difficult to control the melt temperature to be uniform everywhere in the casting crucible or ladle. In practice, the temperatures of the melt at the wall, center, top and bottom of the casting crucible or ladle are not the same. So, with a low superheat temperature, there is a high risk of forming solidified sheets or skins of metals at locations with the lowest temperatures first. These large skins will then flow with the melt into the die cavity resulting in low fluidity and shrinkage feeding problems. Consequently, this casting process causes defects and part rejects. The solidified skins from the crucible or ladle walls also pose other problems in the production process. If not properly removed, these solidified skins will build up at the wall of the crucible. So, there must be a means or process to remove them, which will increase the production cost. With these problems, it is not practical to cast metals with a low superheat temperature if the process is not properly modified and controlled. Accordingly, it would be desirable have a process which prepares molten metals before casting with low to zero superheat. In certain aspects of this invention, processes are provided to achieve these conditions.